Formant frequency fluctuation as an index of motor steadiness in the vocal tract

Involuntary movement of the articulatory structures can interfere with the accurate placement of the articulators during consonant production and may also result in distortion of vowel quality. An acoustic method was used to assess motor steadiness in the vocal tract musculature superior to the glottis during vowel production by five subjects with abnormal involuntary orofacial movements associated with tardive dyskinesia and 10 normal subjects. A linear predictive coding technique of spectral analysis yielded formant frequencies from the sustained productions of /a/. Based on the premise that changes in vocal tract configuration can be measured as changes in formant frequency, the sequential segment-to-segment fluctuations of the second formant frequency of these vowel samples were computed and used as an index of motor steadiness. Results showed that formant frequency fluctuation measures for four of the five tardive dyskinetic patients were substantially larger than those of the normal subjects, indicating a reduction of motor steadiness in these four subjects. Factors influencing the validity of this procedure and implications for its use are discussed.